Molecular hydrogen diagnostic diagram to classify the emission at the nucleus in red and in emission spots sp(A) in green and sp(B) in blue. Line ratios are H₂(2−1)S(1)/H2(1−0)S(1) and H₂(1−0)S(3)/H₂(1−0)S(1). The curve represents thermal emission at 1000–3000 K. Horizontal stripes are thermal UV excitation models by Sternberg & Dalgarno (1989). Vertical stripes are non-thermal models by Black & van Dishoeck (1987). The area of X-ray heating models by Draine & Woods (1990) is marked by an open triangle (yellow). The open turquoise circle marks the region of the shock model by Brand et al. (1989).